The present invention is directed to testing of circuit boards. In particular, it is directed to verifying that individual integrated circuits are in their proper locations on the circuit board and are oriented properly.
Circuit-board manufacturers typically test their boards at a variety of levels. In-circuit tests determine whether individual integrated circuit function properly on the board. Functional tests determine whether the integrated circuits work together to perform the circuit board's intended function. Simpler tests usually precede in-circuit and functional testing. Continuity tests, for instance, verify that electrical continuity exists between points that are intended to be connected, and shorts tests insure that there are no short circuits between nodes that should be isolated.
In addition to these tests, it is also desirable to perform tests to determine whether integrated circuits of the proper types are properly positioned and oriented on the circuit board. Initial detection of a missing or misoriented integrated circuit or an integrated circuit of the wrong type eliminates significant diagnostic effort at a later stage in the testing process. However, many methods for making the necessary determinations automatically have proved unsatisfactory.
The method that is conceptually the most straightforward is to include in the in-circuit test of the integrated circuit an initial diagnostic sequence of stimuli and measurements that would make an initial determination of proper integrated-circuit placement and thus eliminate further testing if the integrated circuit is determined to be missing or in an improper orientation. In the past, this approach has proved disadvantageous because a symptom of improper orientation is often that the integrated circuit burns or even explodes soon after power is applied to it.
Many manufacturers have therefore turned instead to power-off tests, in which the tester measures passive impedances between terminals of the integrated circuit without applying power to it. However, integrated-circuit manufacturers do not ordinarily include such impedances in their specifications, and such impedances can vary greatly between integrated circuits of the same type supplied by different sources. As a result, such tests tend to be unreliable, so they are often omitted or ignored in practice.
It is accordingly an object of the present invention to test for proper type, location, and orientation in such a manner as to provide reliable results without subjecting the circuit elements to a significant risk of damage. Another object is to test circuit boards in an improved manner.